The field of the invention is systems and methods for radiation therapy. More particularly, the invention relates to systems and methods for designing a radiation treatment plan for a radiation therapy system, such as an intensity modulated arc therapy system (“IMAT”).
In practice, intensity modulated radiation therapy (“IMRT”) can be delivered in two ways: via a fixed gantry and via a rotatable gantry. Fixed-gantry IMRT is achieved by delivering overlapping fields from a small number of fixed beam directions. Rotational IMRT is achieved by dynamically changing collimator aperture shapes as the gantry moves around the patient in one or more sweeps along an arc during radiation delivery. Intensity-modulated arc therapy (“IMAT”), a rotational IMRT delivery technique, was first proposed as an alternative to tomotherapy. In tomotherapy, a narrow multileaf collimator (“MLC”) is used to control aperture shapes as the radiation source rotates about the patient. At the same time, the couch on which the patient rests is translated. In contrast, IMAT is performed with a conventional linear accelerator, and the large set of tungsten leaves of the MLC is used to change the “shape” of the aperture as the gantry rotates during delivery.
In IMAT, the treatment is delivered along arcs with a single sweep or multiple sweeps, each with a start and stop position, and the patient remains stationary during the delivery process. The MLC field shape changes continuously during gantry rotation between a collection of specified apertures. Typically, apertures are specified at a set of equally-spaced angles that may be referred to as an “angle grid.” An estimate of the radiation delivered to the patient can then be made by summing the radiation delivered from the angle grid. Note that multiple overlapping arcs provide multiple apertures at each angle and, thereby, yield a modulated intensity distribution from each delivery angle. Thus, an approach to IMAT treatment planning that reflects procedures typically used in IMRT treatment planning is to compute a collection of idealized real-valued fluence matrices over the angle grid, and then approximate these matrices by weighted sums of binary matrices corresponding to the collection of apertures at each angle. These weights and apertures are typically chosen to minimize the approximation error subject to delivery constraints.
A key advantage of IMAT is that the delivery is achieved using a conventional linear accelerator and a conventional MLC. Therefore, IMAT treatments can be delivered using existing equipment in most radiation oncology departments. However, it should be noted that to deliver IMAT plans, the linear accelerator must be equipped with the capability for dynamic delivery. Elekta volumetric arc therapy (“VMAT”) and Varian Medical Systems rapid arc VMAT are two commercially available IMAT systems that, generally, use only a single sweep through one arc for treatment delivery. In some instances, additional sweeps may be used to treat cases involving complex geometries.
Traditionally, IMRT treatment plans are generated with either a two-step process, or a direct aperture optimization (“DAO”) process. In two-step processes, the matrices of beamlet (also called pencil beam) intensities for the set of specified beam directions are optimized. The resulting optimized intensity matrices or maps are then sent to a leaf-sequencer that determines, via the solution of segmentation problems, the set of deliverable MLC shapes and intensities that are combined to approximate each optimized intensity map. It is in this step where any delivery constraints of the treatment unit are enforced. The output of the two-step procedure is a set of MLC aperture shapes and their corresponding weights (known clinically as monitor units) that are referred to as a “segmentation.” In contrast, DAO methods do not utilize optimized intensity maps, but instead generate sets of apertures that are appropriate for each angle and then seek to weight, and sometimes modify, these apertures in order to obtain a desirable dose distribution.